Method of attaching printed circuit boards within an electronic module housing

ABSTRACT

Reflow soldering of a variety of circuit boards ( 9, 11, 15 ) in a variety of sizes and shapes to assigned locations on the base or carrier ( 13 ) of the electronic module housing ( 3 ) is simplified by eliminating custom made metal blocks previously used to clamp the circuit boards against the carrier metal. Instead, the solder-backed circuit boards are placed in assigned positions in the module housing and the inside volume of that housing is filled ( 22 ) with particulate, such as small beads ( 17 ), covering the circuit boards, but leaving the edges of the upstanding metal shields ( 5  and  7 ) visible. A plate ( 21 ) backed foam sheet ( 19 ) is placed over the module housing ( 24 ) and clamped down ( 26 ), pressing against the beads. The clamped assembly is then heated ( 28 ) to reflow the solder, soldering the circuit boards in place.

FIELD OF THE INVENTION

[0001] This invention relates to manufacturing electronic modules, and,more particularly, to a method and apparatus for efficient re-flowsolder bonding of multiple populated circuit boards of a variety ofdifferent sizes and thickness within respective compartments in themodule housing carrier, without use of custom-made clamping blocks.

BACKGROUND

[0002] Electronic equipment is often modularized, particularly that inthe field of RF communication and at microwave frequencies and higher.That is, various electronic circuits are packaged together in a moduleand the module fits within (and is inserted into) a rack or othercabinet, alongside other modules, which collectively form an electronicsystem. Each module contains electrical connectors that plug into matingconnectors in the rack or cabinet, enabling the various modules tocommunicate with other circuits in other modules via transmission linesin the respective rack or cabinet, and/or receive electrical power foroperation. Typically, the module is constructed of a strong shallowrectangular metal frame, somewhat resembling a picture frame inappearance, and containing top and bottom metal walls, referred to asthe cover and the carrier, that fasten to the frame and cover therectangular window-like openings on top and bottom sides. Collectively,the foregoing frame and other components are often referred to as themodule housing.

[0003] The metal walls and frame of the module housing shield theelectronic circuitry inside the module from outside RF interference and,conversely, prevent RF from escaping from the module and causinginterference with external apparatus. The housing carrier or wall alsoserves as a thermal transmission path, allowing heat from the circuitboards, internally generated inside the module housing, to pass to aheat sink or otherwise dissipate in the environment.

[0004] The module houses a variety of populated printed circuit boardscarrying the electronic circuits that together defines the purpose ofthe particular module in the electronic system. Those circuit boards,populated with electronic chips and other electronic components, areplaced in positions about the available space in the rectangular area onthe carrier wall inside the module housing that are assigned by thecircuit designer. Since the electronic functions of the circuit boardstypically differ from one another, individual circuit boards may vary insize and height, but all fit within the available space on the modulehousing carrier and within the internal volume of the module housing.

[0005] With the top wall or cover of the module housing removed, theinside region appears compartmentalized, a second physicalcharacteristic of the internal region. The inside region is divided by avariety of metal walls upstanding from the bottom wall and theindividual circuit boards fit inside respective regions defined by twoor more associated metal walls. The upstanding walls may appear in avariety of lengths and some contain bends, producing open and/or closedcompartments in a variety of sizes. The metal walls are attached to andextend from the bottom carrier wall up to the plane of the inner surfaceof the top cover so that, with the top wall, the module cover, fastenedin place, the upstanding walls, cover and bottom carrier define a numberof metal three-dimensional cavities or regions, each of which contains acircuit board that is populated with electronic components or microstriplines.

[0006] The upstanding metal walls of the module serve as RF shielding toprevent RF energy as may be generated in and radiated from a givencircuit board from propagating to another circuit board by an undesiredroute that could cause improper functioning of the electronic circuitsor render the electronic circuits dysfunctional. The complexity andvariety of two-dimensional shapes defined by the internal walls inside atypical module is visible in the isometric view of a typical modulepresented in FIG. 1, which may be inspected briefly.

[0007] As those skilled in the art recognize, individual circuit boardsare typically fastened to the bottom wall of the module by solder, whichholds the respective boards in place and places the electrical ground ofthe circuit board in common with the metal walls of the module housing.The bottom side of each printed circuit board, which may be a laminate,contains a metal outer surface, typically a solder coating. That metalouter surface is pre-tinned as required by the conventional solderreflow process. By pressing the circuit board against the metal bottomwall of the module and re-flowing the solder, that is, heating thesolder (and other components in the vicinity) to the eutectictemperature of the solder, after which the heat is removed, the solderresolidifies and bonds the printed wiring board in place. This lastbriefly summarized step is one that produces the greatest expense andeffort in the assembly process, which the present invention addressesand improves upon.

[0008] In existing practice pressing a circuit board against the bottomwall of the module during the solder reflow process requires use of ametal block, often formed of titanium or aluminum. The circuit board isclamped between the metal block and the metal wall and then heat isapplied. With many circuit boards in a module, there must be manyblocks, all of which are clamped for the solder reflow process.

[0009] The metal block must fit in the space occupied by the associatedcircuit board, and between any of the upstanding metal walls adjoiningthe respective circuit board. The metal block must also be of sufficientheight to reach the top of the module and be accessible to the clamp. Asearlier explained (and as is apparent from FIG. 1) each of thecompartments of the module's real estate may be of a different size andshape. Hence, the associated metal block for a respective region must beindividually machined to shape. A typical module may require thirty ormore individually machined metal clamping blocks. Thus, once the layoutof the module is finalized by the module designer and ready formanufacture, the manufacturing engineers must prescribe the size of theblocks needed for the manufacturing process.

[0010] The blocks need to be machined to shape, typically to tolerancesof thousandths of an inch. As those skilled in the art appreciate thefabrication of the metal blocks is a time consuming procedure and makesmanufacturing set-up expensive. Moreover, since volume manufacturing ofcommunications equipment using such modules may be limited, the cost ofthe foregoing set up on a per unit basis is quite high. Since anelectronic system contains a number of different modules of differentfunction, and, hence, possess different layouts, a great many differentmetal blocks must be machined to shape, perhaps hundreds of such blocks.And pity the engineer who determines that it is necessary to change thelayout once the electronic module design has been released formanufacture.

[0011] A principal reason for using a clamping procedure during thesolder reflow process is to ensure that the entire bottom surface ofeach circuit board is soldered to the metal wall, particularly aroundthe edges of the circuit board. As example, should the side edge of acircuit board be uplifted from the board due to ineffective soldering,leaving a gap, the gap increases the length of the ground path to thenext circuit board in the electronic circuit in the module. If the RFoutput from the circuit board is propagating a high frequency microwavesignal to an adjacent microstrip line or another circuit board, asexample, the ground path for return current is increased, undesirablyintroducing attenuation of the signal. Moreover, due to the metal sidesof the wall and circuit board, the gap forms a microwave cavity that maybe resonant at the frequency of RF being propagated from the circuitboard, which may result in oscillations or signal path power losses.

[0012] Use of metal blocks is also slightly disadvantageous to thesolder reflow process. Since the metal block is heat transmissive, theblock conducts heat away from the solder, requiring more heat be appliedthan otherwise.

[0013] As an advantage, the present invention obsoletes metal clampingblocks and renders those blocks unnecessary for the solder reflowprocess in module fabrication. The printed circuit boards may be solderbonded to the bottom wall of the module by reflowing the solder bypressing the printed circuit board against that wall and reflowing thesolder, but without use of metal clamping blocks.

[0014] Accordingly, an object of the invention is to enhance theefficiency of the process of manufacturing electronic modules and reducemanufacturing cost.

[0015] A further object of the invention is to reduce the time and costof set-up required for the manufacture of electronic modules, and, morespecifically for reflow soldering of circuit boards in place in themodule housing.

[0016] And a still further object of the invention is to press printedcircuit boards against the metal surface during reflow soldering ofthose circuit boards to the metal surface without using metal clampingblocks.

SUMMARY OF THE INVENTION

[0017] In accordance with the invention, solder-backed circuit boardsare placed in assigned positions on the carrier metal and then theinside volume of the electronic module housing is filled to the top withparticulate, such as small beads, while leaving the edges of anyupstanding metal shields in the inside volume visible. A plate backedsheet of foam material, e.g. a foam sheet, is placed over the modulehousing and is then clamped down, pressing against the particulate, and,through the particulate, pressing the confined circuit boards againstthe carrier metal. The clamped assembly is then placed in a heatingchamber or other reflow processing station and is heated to reflow thesolder on the back of the circuit boards. The heating is withdrawn andthe assembly is allowed to cool down, completing the reflow soldering.The particulate is removed from the housing interior (and may bereused), completing the soldering process.

[0018] In accordance with a more specific aspect of the invention theparticles of the particulate are glass spheres. And, in accordance witha further specific aspect of the invention, the particulate is ofmaterial that is no better than poor in thermal conductivity.

[0019] The foregoing method allows soldering of a variety of circuitboards of a variety of size and shape to assigned locations on the baseor carrier metal of the electronic module to be completed all at once.By eliminating custom made metal blocks previously used to clamp thecircuit boards against the carrier metal the complex and expensive workto fabricate those metal blocks to shape in setting up for solder reflowprocessing is avoided; and the assembly of circuit boards to theelectronic module housing is greatly simplified.

[0020] The foregoing and additional objects and advantages of theinvention, together with the structure characteristic thereof, whichwere only briefly summarized in the foregoing passages, will become moreapparent to those skilled in the art upon reading the detaileddescription of a preferred embodiment of the invention, which follows inthis specification, taken together with the illustrations thereofpresented in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the drawings:

[0022]FIG. 1 is an isometric view of an electronic module with the coverof the module housing removed to expose the general internal structureof the module and the contents of the interior volume;

[0023]FIG. 2 is a side section view of the principal apparatus used toaccomplish the invention; and

[0024]FIG. 3 is a process diagram of the new method that incorporatesthe apparatus of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Reference is first made to FIG. 1, which provides an isometricview of a module 1 that is typical of the prior art construction, itbeing understood that the product produced by the practice of theinvention is the same as the prior art. The module, shown raised in avertical position, contains a housing 3 formed of a generally narrowrectangular metal frame, defining a relatively large rectangular region,a metal bottom wall or, as variously termed carrier, not visible in thefigure, located on the far side of the housing that covers the largerectangular region from the opposite side or underside. A metal coverpanel, not illustrated, that closes the open end of the housing, isremoved so that the internal contents of the module housing may beviewed. For purposes of this description the open or uncovered side ofthe module is referred to as the top or upper side, and the carrier orbottom wall is referred to as the bottom side of the housing. As oneappreciates, a completed module may be mounted in a rack, eitherhorizontally or vertically. Even if mounted vertically as shown in thefigure, it should be understood that the right side of the modulehousing in the figure is being referred to herein as the top or upperside for reasons which become more apparent later in this description.

[0026] The inside of housing 3 contains a large number of metal walls,such as 5 and 7, only two of which are identified by number. Those wallsdivide the internal rectangular region of the housing into variouscompartments, some of which may be open, and others closed. The metalwalls are attached to or are formed integrally in the bottom carrier ofthe housing and are of a height that extends to the plane defined by thehousing edges at the upper end of the housing. That permits the top edgeof the walls to contact the metal surface of the cover for the bestpossible RF shielding between compartments.

[0027] Each compartment contains a circuit board that is sized to fitinto the compartment specified by the designer, an assigned location,such as circuit boards 9 and 11, only two of which are identified bynumber. The compartments (and the associated circuit boards) definesub-regions of odd sized generally rectangular shaped areas in a varietyof sizes and shapes. As was earlier described in the background to thisdescription, those sizes and shapes are dictated by the necessities ofthe functional and mechanical design of the electronic module.

[0028] As described in the background the circuit boards are pretinnedfor soldering and are placed in the assigned compartments. In the priormethod for soldering those boards, the custom fabricated metal blocksare inserted into the respective assigned compartments, and are clampedagainst the circuit boards, forming an assembly that is then introducedinto the heating of the conventional solder reflowing process. Althoughthe illustration serves as background to the process describedhereafter, one readily appreciates the enormous complexity, time andexpense involved in preparing the many different metal blocks for theprior art soldering process for just this single module.

[0029] Reference is made to the partial section view of FIG. 2, whichshows the principal components used to practice a preferred embodimentof the method invention and the relationship of those components to themodule housing and circuit boards during the reflow soldering process.The bottom carrier 13 of module housing 3, partially illustrated, isshown supporting two of the upstanding metal walls, 5 and 7, that form acompartment within the housing. A circuit board 15 is disposed in placein the assigned compartment defined in part by walls 5 and 7, ready forthe reflow soldering process that solders that circuit board to thebottom carrier 13.

[0030] Small spheres or spheroids 17, referred to herein as beads,overlie circuit board 15 and fill the formed compartment, up to the topedge of the upstanding walls. A sheet 19 of a compressible material,commonly referred to as high temperature gasket material, or other likematerial, partially illustrated, is placed over the top end of themodule housing. That sheet is backed (or overlain) by another sheet orplate 21 of more rigid material, such as an aluminum plate 21.

[0031] A clamp, not illustrated, provides a clamping force to the formedsandwich of components. By applying a squeezing or clamping force, F,between the aluminum plate 21 and bottom carrier 13, foam sheet 19 ispressed against the edges of walls 5 and 7, and yields or compressesalong those edges, while the other portions of the foam sheet, locatedbetween the walls, presses against beads 17. In doing so, a portion ofthe foam sheet overlying the area between the walls protrudes slightlyinto the formed compartment. As one appreciates, although the foam sheetis sufficiently compressible and gives in regions thereof pressedagainst the edges of the relatively thick strong metal walls the foam issufficiently rigid so as not to give significantly under the reactiveforce exerted by the beads, and holds the beads under pressure.Responding to the clamping force, beads 17 distribute evenly and applypressure evenly over the entire circuit board 17 and particularly theedges of the circuit boards where soldering is most critical.

[0032] Although only a portion of the module housing and a singleinternal compartment is illustrated in FIG. 2, it should be realizedthat the beads fill all compartments within the module housing and thatthe metal backed foam sheet covers the entire upper end of modulehousing 3. Hence what is illustrated in FIG. 2 is repeated over theentire inside of the module housing, and the clamping action described,occurs in all compartments concurrently.

[0033] Although the invention should become evident from the foregoingdescription, for completeness reference is made to FIG. 3, whichillustrates the steps of the circuit board mounting process. Assumingone is given a module housing and the circuit boards for that housing, afirst step is to place the circuit boards through the uncovered upperend of the housing and into the assigned locations on the carrier bottominside the module housing, as illustrated at 20. Next beads aredeposited inside the module until the entire inside is filled with thebeads as represented at 22. A straight spatula, like a cake spatula, maybe wiped across the upper edge of the housing to ensure that the glassbeads do not form a mound at any particular location or deposit on thetop edge of any of the internal upstanding shielding walls.

[0034] The metal plate backed foam sheet is then placed on top of themodule to cover the open end, as represented at 24, and a clamping forceis applied between the metal plate and the bottom carrier of the housingas at 26 to form a clamped assembly, thereby squeezing the beads againstthe circuit board and holding the circuit board flat against the bottomcarrier. The clamped assembly is then placed in a heating chamber forreflow soldering as at 28. Reflow is conventional in operation. The heatcauses the solder on the rear side of the circuit board to attain theeutectic temperature and the solder melts and flows into the intersticesin the adjoining metal of the bottom carrier. Thereafter the heating isterminated (or the clamped assembly is removed from the oven) and theclamped assembly is permitted to cool down, whereby the solderresolidifies and produces an attachment between the circuit boards andthe carrier.

[0035] Once cooled, it is desirable to remove the beads from the modulehousing. The beads may be evacuated by a vacuum apparatus, essentially avacuum cleaner that draws out the beads and deposits them in acontainer, from which the beads can later be retrieved and reused. Ifnecessary, any remnants of beads found after vacuuming may be removed bybrushing them out or agitating the housing and dumping them out. Anyresidue from the beads may be removed by conventional solvent cleaning.The foregoing concludes the process. For completion of the electronicmodule, the cover is installed in place to cover the open end.

[0036] Reference is again made to FIG. 2. Beads 17 in this embodimentare small spheres or spheroids in shape, are sufficiently rigid towithstand the clamping force exerted by the foam sheet withoutsignificant deformation, and are capable of withstanding the effects ofthe incident heating encountered in the solder reflow operation withoutsignificant deformation or conduction of heat. Preferably the beadmaterial should also be a relatively poor thermal conductor according toindustry standards, that is, poor relative to other materials that maysatisfy the preceding characteristics. The poor thermal conductivityminimizes escape of heat from the circuit board during reflow soldering(e.g., heating) that could interfere with soldering. The foregoingreference to a poor thermal conductor is understood to include withinthe meaning of that term a thermal non-conductor, which is the ideal.Preferably, the material that forms the beads should also be inorganic,so as not to adversely affect the circuit board. At present, glass isthe preferred material, and, more specifically borosilicate glass, suchas manufactured by the MO-SCI Corporation. As those skilled in the artappreciate other materials may be substituted.

[0037] The beads should be small enough in diameter relative to thecavity so that a large number of beads is necessary to cover the surfaceof the circuit board. In that way the beads should distribute theclamping force more evenly over the surface of the associated printedcircuit boards. In one practical embodiment the beads are 0.10 inches indiameter.

[0038] In one practical embodiment foam sheet 19 may be constructed froman expanded silicone elastomer material marketed by the Boyd Corporationand may be approximately one-half inch (1.27 cm) thick. In analternative construction in which a more stiff foam material is desired,that might not easily compress against the edges of the upstanding walls5 and 7, then the foam sheet should be modified to include grooves of apredetermined depth. For such an alternative a layout is made of all ofthe walls contained within the module. The foam sheet is then grooved inthe image of that wall layout to a short depth. The outer dimensions ofthe foam sheet are such as to fit within the outer walls of the housing.Then when the sheet is overturned and aligned so that the groovesreceive the edges of the walls, such as 5 and 7, the sheet may bepressed against the beads without having to overcome the resistance ofthe wall edges.

[0039] The invention is not to be construed in a limited sense. As thoseskilled in the art appreciate from an understanding of the foregoingdescription, the invention is not limited to the shape and material ofthe beads, the particles of the particulate used in the foregoingembodiment. Spheres and glass are at present the preferred shape andmaterial, since glass spheres may be easily manufactured at reasonablecost. It should be clear that other shapes, as example, spheroidal orellipsoidal, may be substituted. Further, whatever shape is chosen forthe particulate, the particle may be made of glass or other materialhaving the low thermal conductive or non-conductive property describedearlier. Even pebbles or rice-shaped particles of borosilicate glasscould be used in alternative embodiments of the method. All such shapedmaterials appear to function in the described method as equivalents.

[0040] As the reader should appreciate, the term selected as generic toall such beads is “particulate”. Webster's New Collegiate dictionarydefines particulate as being “of or relating to minute separateparticles”, and defines a particle as “a relatively small . . . discreteportion of something”. Thus, as used herein, the term particulatesubsumes all possible shapes found to function in the described processto produce the described result.

[0041] It is believed that the foregoing description of the preferredembodiments of the invention is sufficient in detail to enable oneskilled in the art to make and use the invention without undueexperimentation. However, it is expressly understood that the detail ofthe elements comprising the embodiment presented for the foregoingpurpose is not intended to limit the scope of the invention in any way,in as much as equivalents to those elements and other modificationsthereof, all of which come within the scope of the invention, willbecome apparent to those skilled in the art upon reading thisspecification. Thus, the invention is to be broadly construed within thefull scope of the appended claims.

What is claimed is:
 1. A method of assembling a circuit board to thecarrier wall of the housing for an electronic module, said circuit boardcontaining a bottom layer of a solder material, comprising the steps of:placing the circuit board onto a predetermined location on the carrierwall with said bottom layer of said circuit board resting on saidcarrier wall; depositing particulate on the circuit board anddistributing said particulate over at least the upper surface of saidcircuit board; applying a force against said particulate, wherein atleast some particles of said particulate press said circuit boardagainst said carrier wall; and reflowing said solder while applying saidforce against said particulate to attach said circuit board to saidcarrier wall; removing said force from said spheres to release pressureon said particulate and said circuit board; and removing saidparticulate.
 2. The method of assembling a circuit board to the carrierwall of an electronic module as defined in claim 1, wherein said step ofapplying a force against said particulate includes the steps of: placinga layer of foam material over said particulate; placing a layer of rigidmetal over said layer of foam material; applying a clamping forcebetween said layer of rigid metal and said carrier wall of said housingto press said foam material against said particulate.
 3. The method ofassembling a circuit board to the carrier wall of the housing of anelectronic module as defined in claim 1, wherein said step of reflowingsaid solder includes the steps of: placing said particulate, saidcircuit board and said housing into a vapor soldering chamber, and,while maintaining said applied force against said particulate, heatingsaid vapor soldering chamber to raise the temperature of said solder tothe eutectic temperature of said solder; and then terminating saidheating.
 4. The method of assembling a circuit board to the carrier wallof the housing of an electronic module as defined in claim 1, whereinsaid housing includes side walls, said side walls together with saidcarrier wall defining a cavity; and wherein said step of depositingparticulate on the circuit board includes depositing said particulate ina quantity sufficient to fill said cavity.
 5. The method of assembling acircuit board to the carrier wall of the housing of an electronic moduleas defined in claim 1, wherein said particulate comprises materialhaving poor thermal conductivity.
 6. The method of assembling a circuitboard to the carrier wall of the housing of an electronic module asdefined in claim 1, wherein said material having poor thermalconductivity comprises glass.
 7. The method of assembling a circuitboard to the carrier wall of the housing of an electronic module asdefined in claim 1, wherein the particles of said particulate comprisespheres.
 8. The method of assembling a circuit board to the carrier wallof the housing of an electronic module as defined in claim 1, whereinthe size of the particles of said particulate is substantially less thanthe length or width of said circuit board.
 9. The method of assembling acircuit board to the carrier wall of an electronic module as defined inclaim 2, wherein said foam material is compressible, whereby said foammaterial may compress in locations that press against edges of anyupstanding walls in said cavity while also pressing against saidparticulate.
 10. The method of assembling a circuit board to the carrierwall of an electronic module as defined in claim 1, wherein said step ofremoving said particulate comprises: applying a vacuum nozzle to saidcavity to suck said particulate into a canister.
 11. A method ofassembling individual circuit boards to the housing of an. electronicmodule, said circuit boards containing a bottom layer of a soldermaterial and being variegated in size and shape, and said housing ofsaid electronic module containing a bottom wall defining a cavity andcontaining upstanding internal walls defining a variety of compartmentswithin said cavity, said individual circuit boards being of a size andshape within respective ones of said compartments, comprising the stepsof: placing said circuit boards within respective compartments insidesaid cavity with said bottom layer of said circuit boards resting onsaid bottom wall of said module; filling said cavity with particulate,wherein said circuit boards are covered by said particulate; pressingagainst said particulate, wherein at least some portion of saidparticulate presses said circuit boards against said bottom wall of saidmodule; and reflowing said solder while maintaining said particulatepressed against said circuit boards to attach said circuit boards tosaid bottom wall of said module.
 12. The method of assembling circuitboards to the bottom wall of an electronic module as defined in claim11, wherein said particles of said particulate comprise spheres.
 13. Themethod of assembling circuit boards to the bottom wall of an electronicmodule as defined in claim 11, wherein said step of pressing includesthe steps of: placing a layer of foam material over said particulate;placing a layer of rigid metal over said layer of foam material;applying a clamp between said layer of rigid metal and said electronicmodule to squeeze said layer of foam material against said particulate.14. The method of assembling circuit boards to the bottom wall of anelectronic module as defined in claim 13, wherein said foam materialcomprises a compressibility characteristic whereby said foam materialyields in locations where pressed against the edges of said compartmentwalls while sufficiently stiff enough to concurrently press against saidparticulate.
 15. The method of assembling circuit boards to the bottomwall of an electronic module as defined in claim 13, wherein said foammaterial is relatively stiff, and wherein one side of said layer of foammaterial includes grooves for receiving the edges of said compartmentwalls to permit portions of said layer to press against said glass ballswithout interference of said compartment walls.
 16. A method ofassembling a circuit board to the carrier wall of the housing for anelectronic module, said housing including side walls with said sidewalls together with said carrier wall defining a cavity, and saidcircuit board containing a bottom layer of a solder material, comprisingthe steps of: placing the circuit board onto a predetermined location onthe carrier wall with said bottom layer of said circuit board resting onsaid carrier wall; depositing glass balls of borosilicate glass in saidcavity in a quantity sufficient to fill said cavity and cover said uppersurface of said circuit board; applying a force against said glassballs, wherein at least some glass balls press said circuit boardagainst said carrier wall, said step of applying a force against saidglass balls including the steps of: placing a layer of foam materialover said glass balls, said foam material comprising an expanded siliconelastomer; placing a layer of rigid metal over said layer of foammaterial; applying a clamping force between said layer of rigid metaland said carrier wall of said housing to press said foam materialagainst said glass balls; and reflowing said solder while applying saidforce against said glass balls to attach said circuit board to saidcarrier wall, said step of reflowing said solder including the steps of:placing said glass balls, said circuit board and said housing into avapor soldering chamber, and, while maintaining said applied forceagainst said glass balls, heating said vapor soldering chamber to raisethe temperature of said solder to the eutectic temperature of saidsolder; and then terminating said heating; removing said force from saidglass balls release pressure on said glass balls and said circuit board;and removing said glass balls.
 17. Clamping apparatus for clampingcircuit boards to the carrier wall of the housing of an electronicmodule during reflow solder processing to attach said circuit boards tosaid carrier wall, comprising: a particulate, said particulate beingsufficient in number to fill said housing; a sheet of foam material,said sheet being sufficient in size to cover an open end of saidhousing; a rigid plate, said rigid plate being sufficient in size tocover said sheet of foam material.
 18. The clamping apparatus as definedin claim 17, wherein said spheres comprise wherein the particles of saidparticulate comprise spheres.
 19. The clamping apparatus as defined inclaim 18, wherein said spheres comprise: glass balls.
 20. The clampingapparatus as defined in claim 19, wherein said glass of said glass ballscomprises borosilicate glass and wherein said foam material comprises anexpanded silicon elastomer.
 21. The clamping apparatus as defined inclaim 17, wherein said sheet of foam material includes a surfacecontaining grooves.